This application is a 371 of PCT/GB00/03805, filed Oct. 4, 2000.
This invention relates to a process for the co-production of C4 compounds, more specifically maleic anhydride, butane-1,4-diol, xcex3-butyrolactone, and tetrahydrofuran, from a hydrocarbon feedstock selected from C4 hydrocarbons and benzene.
Maleic anhydride can be produced by vapour phase oxidation of a hydrocarbon feedstock, such as benzene, mixed C4 olefins, or n-butane, in the presence of a partial oxidation catalyst.
Depending on the nature of the feedstock a supported promoted vanadium pentoxide catalyst is typically used, while the reaction temperature is usually from about 350xc2x0 C. to about 500xc2x0 C. and the reaction pressure is from about 105 Pa to about 3xc3x97105 Pa. A substantial excess amount of air may be used in order to stay outside the explosive limits. The contact time is about 0.1 s. Alternatively it is possible, according to more modern practice, to design the plant so that satisfactory safe operation can be achieved, despite the fact that the feed mixture of air and hydrocarbon feedstock is within the flammable limits.
One design of reactor for such partial oxidation reactions comprises a tubular reactor including vertical tubes surrounded by a jacket through which a molten salt is circulated in order to control the reaction temperature. However, other designs of reactor can be used instead, including fixed bed reactors, fluidised bed reactors, or moving bed reactors.
In each case a hot vaporous reaction mixture is recovered from the exit end of the reactor which comprises maleic anhydride vapour, water vapour, carbon oxides, oxygen, nitrogen, and other inert gases, besides organic impurities such as acetic acid, acrylic acid, and unconverted hydrocarbon feedstock.
It is usual to recover and purify the maleic anhydride from this dilute reactor effluent stream in up to four steps. First, in an optional step, some conventional processes condense out part of the maleic anhydride by cooling the reactor effluent stream, typically to about 150xc2x0 C. using a steam-producing heat exchanger and then cooling it further to about 60xc2x0 C. by cooling it against water, in order to condense part of the maleic anhydride, typically about 30% to about 60% of the maleic anhydride present. Only partial condensation is effected because of the presence of water which reacts with maleic anhydride in the reactor effluent stream to form maleic acid, which may in turn isomerise to fumaric acid. Maleic acid has a melting point of 130xc2x0 C., while fumaric acid has a melting point of 287xc2x0 C., both of which are much higher than that of maleic anhydride (52.85xc2x0 C.). As a result there is a tendency for deposits of solid maleic acid and fumaric acid to build up on the heat exchanger surfaces which require periodic removal, typically by use of water and/or sodium hydroxide solution which yields an aqueous solution that contains fumaric acid and maleic acid or their sodium salts and requires effluent treatment.
A second step that is conventionally used is to absorb essentially all of the remaining maleic anhydride from the effluent stream. The remaining gaseous effluent can then be vented to the atmosphere, possibly after incineration of carbon monoxide, unconverted hydrocarbon, and other organic compounds contained therein. In this absorption step an organic solvent can be used. Alternatively an aqueous solution can be used as the absorbent, in which case the maleic anhydride is mainly hydrolysed to form maleic acid.
Scrubbing with water or with an aqueous solution or slurry is described, for example, in U.S. Pat. No. 2,638,481. A disadvantage of such a procedure, however, is that some of the maleic acid is inevitably isomerised to fumaric acid. The byproduct fumaric acid represents a loss of valuable maleic anhydride and is difficult to recover from the process system since it tends to form crystalline masses which give rise to process fouling problems.
Because of this isomerisation problem a variety of other anhydrous organic solvents have been proposed for absorption of maleic anhydride from vaporous streams, for example, dibutyl phthalate (British Patent Specifications Nos. 727,828, 763,339, and 768,551), dibutyl phthalate containing up to 10 weight % phthalic anhydride (U.S. Pat. No. 4,118,403) normally liquid intramolecular carboxylic acid anhydrides, such as a branched chain C12-15-alkenyl substituted succinic anhydride (U.S. Pat. No. 3,818,680), tricresyl phosphate (French Patent Specification No. 1,125,014), dimethyl terephthalate (Japanese Patent Publication No. 32-8408), dibutyl maleate (Japanese Patent Publication No. 35-7460), high molecular weight waxes (U.S. Pat. No. 3,040,059), diphenylpentachloride (U.S. Pat. No. 2,893,924), high boiling aromatic hydrocarbon solvents, such as dibenzylbenzene (French Patent Specification No. 2,285,386), dimethylbenzophenone (U.S. Pat. No. 3,850,758), polymethylbenzophenones, at least a portion of which contain at least 3 methyl groups, (U.S. Pat. No. 4,071,540), water-insoluble tertiary amines (U.S. Pat. No. 4,571,426), dialkyl phthalate esters having C4 to C8 alkyl groups and a total of 10 to 14 carbon atoms in both alkyl groups (U.S. Pat. No. 3,891,680), and esters of cycloaliphatic acids, for example dibutyl hexahydrophthalate (South African Patent Specification No. 80/1247).
A third step that is conventionally used is to recover the resulting solution of maleic anhydride or maleic acid from the absorbent. When the absorbent is an organic solvent, batch distillation or continuous distillation can be used to recover the maleic anhydride. On the other hand, when the absorbent liquid is water or an aqueous solution, the recovery step must include a dehydration step so as to re-convert the maleic acid back to maleic anhydride. One procedure that is used is to distil the maleic acid solution in the presence of xylene. This not only removes the water but also results in re-formation of maleic anhydride. In either event the elevated temperatures used tend to induce formation of fumaric acid which constitutes a further loss of potential product maleic anhydride.
U.S. Pat. No. 5,069,687 proposes recovery of maleic anhydride from a gaseous mixture by contact with an absorbent, following which water is removed from the enriched absorbent by contacting it with a water adsorbent or with a low humidity stripping gas. Maleic anhydride is then recovered from the dried enriched absorbent.
A growing use for maleic anhydride is in the production of butane-1,4-diol, and its co-products, i.e. xcex3-butyrolactone, and tetrahydrofuran. Direct hydrogenation of maleic anhydride or maleic acid to these C4 compounds is proposed in U.S. Pat. Nos. 3,948,805, 4,001,282, 4,048,196, 4,083,809, 4,096,156, 4,550,185, 4,609,636, 4,659,686, 4,777,303, 4,985,572, 5,149,680, 5,347,021, 5,473,086, and 5,698,749, and in European Patent Publication No. 0373947A.
Esterification of maleic anhydride with an alkyl alcohol to yield a dialkyl maleate followed by hydrogenation of the resulting dialkyl maleate has also been proposed in order to produce butane-1,4-diol, and its co-products, xcex3-butyrolactone and tetrahydrofuran. Hydrogenation in the liquid phase is proposed in British Patent Specification No. 1,454,440. Vapour phase hydrogenation is taught in International Patent Publication No. WO 82/03854. Hydrogenation of a dialkyl maleate in two stages can be carried out as described in U.S. Pat. Nos. 4,584,419 and 4,751,334.
U.S. Pat. No. 4,032,458 proposes esterification of maleic acid with a C2 to C10 alkanol at elevated pressure and temperature followed by a two stage hydrogenation of the resulting dialkyl maleate using a slurry of a copper chromite catalyst and then by distillation.
U.S. Pat. No. 5,478,952 suggests a hydrogenation catalyst which can be used in aqueous solution to hydrogenate, for example, maleic acid, and which consists of a mixture of ruthenium and rhenium on carbon.
Processes and plant for the production of dialkyl maleates from maleic anhydride are described, for example, in U.S. Pat. No. 4,795,824 and in International Patent Publication No. WO 90/08127. This last mentioned document describes a column reactor containing a plurality of esterification trays each having a predetermined liquid hold-up and containing a charge of a solid esterification catalyst, such as an ion exchange resin containing pendant sulphonic acid groups.
The hydrogenation of dialkyl maleates to yield butane-1,4-diol is discussed further in U.S Pat. Nos. 4,584,419, and 4,751,334, and International Patent Publication No. WO 88/00937.
In International Patent Publication No. WO 97/43242 a process is described in which maleic anhydride is absorbed in a high boiling solvent having a boiling point that is at least 30xc2x0 C. higher than that of maleic anhydride at atmospheric pressure, for example dimethyl phthalate. Then the maleic anhydride in the resulting solution is esterified to form the corresponding di-(C1 to C4 alkyl) maleate, which is subsequently stripped from the solution using a hydrogen-containing gas stream to yield a vaporous mixture which is then subjected to vapour phase hydrogenation. A similar procedure in which the esterification step is omitted and the maleic anhydride is stripped from the solution in the high boiling solvent and subjected to vapour phase hydrogenation is described in International Patent Publication No. WO 97/43234. Further materials for use as absorption solvent are taught in International Patent Publications Nos. WO 99/25675 and WO 99/25678.
A further development of such processes is proposed in International Patent Publication No. WO 99/48852; in this development a second high boiling solvent, such as dibutyl phthalate, is used to scrub the off-gas from an absorption step in which maleic anhydride is absorbed from a crude vaporous maleic anhydride stream from a maleic anhydride plant in a first high boiling solvent, such as dimethyl phthalate.
In the prior art processes for production of butane-1,4-diol from maleic anhydride it is normal procedure to utilise a substantially pure maleic anhydride feedstock which contains at most a trace each of light acids (e.g. acetic acid and acrylic acid), of fumaric acid and of maleic acid.
It is an object of the present invention to provide an improved process for the co-production of maleic anhydride and the C4 compounds, butane-1,4-diol, xcex3-butyrolactone, and tetrahydrofuran. It is also an object of the present invention to improve the yield of such C4 compounds from a given quantity of hydrocarbon feedstock and hence to make these compounds more readily available and to reduce the quantity of waste products produced. It is also an object of the present invention to provide a process for the production of the C4 compounds, maleic anhydride, butane-1,4-diol, xcex3-butyrolactone, and tetrahydrofuran from a hydrocarbon feedstock which can be operated in a plant that is more economical to construct and to run than conventional plants.
According to the present invention there is provided a process for the co-production of maleic anhydride and at least one C4 compound selected from butane-1,4-diol, xcex3-butyrolactone, and tetrahydrofuran wherein:
maleic anhydride is produced by steps comprising:
(i) supplying a source of gaseous oxygen and a hydrocarbon feedstock selected from C4 hydrocarbons and benzene to a catalytic partial oxidation zone which contains a charge of a partial oxidation catalyst capable of effecting the partial oxidation of the hydrocarbon feedstock to form maleic anhydride and which is maintained under catalytic partial oxidation conditions;
(ii) recovering from the partial oxidation zone a vaporous reaction effluent stream comprising maleic anhydride, water, unconverted hydrocarbon feedstock, and carbon oxides;
(iii) condensing a part of the maleic anhydride present in the vaporous reaction effluent stream in a condensation zone to form a crude maleic anhydride stream;
(iv) recovering from the condensation step (iii) a residual vaporous stream containing residual amounts of maleic anhydride;
(v) absorbing further maleic anhydride from the residual vaporous stream of step (iv) by absorption in a liquid absorption medium selected from an organic solvent, water, and an aqueous solution;
(vi) recovering from the absorption step (v) a loaded liquid absorption medium; and
(vii) recovering maleic anhydride from the loaded liquid absorption medium; and wherein:
said at least one C4 compound selected from butane-1,4-diol, xcex3-butyrolactone, and tetrahydrofuran is produced by steps comprising:
(viii) providing a C4+ hydrogenation feedstock selected from maleic anhydride, maleic acid, dialkyl maleates, and mixtures of two or more thereof;
(ix) supplying said C4+ hydrogenation feedstock and hydrogen to a hydrogenation zone which contains a charge of a hydrogenation catalyst effective for catalytic hydrogenation of the C4+ hydrogenation feedstock to yield said at least one C4 product and which is maintained under catalytic hydrogenation conditions; and
(x) recovering from the hydrogenation zone a hydrogenation product stream containing said at least one C4 product;
characterised in that crude maleic anhydride condensate of the crude maleic anhydride stream of step (iii) is used as the C4+ hydrogenation feedstock of step (viii) or is used to prepare the C4+ hydrogenation feedstock of step (viii).
The source of gaseous oxygen may comprise substantial amounts of inert gases, such as nitrogen, in addition to oxygen. Air is a convenient source of gaseous oxygen for use in the process of the invention. Hence the vaporous reaction effluent stream of step (ii) may contain nitrogen and oxygen in addition to the other components mentioned. It will often be expedient to cool the vaporous reaction effluent stream of step (ii) prior to attempting to effect condensation in step (iii).
In a preferred process according to the invention all of the crude maleic anhydride reaction product of step (iii) is used as the C4+ hydrogenation feedstock of step (viii) or is used to prepare the C4+ hydrogenation feedstock of step (viii).
If necessary, at least some of the maleic anhydride recovered in step (vii) can be used as or to prepare further C4+ hydrogenation feedstock for use in step (viii).
Alternatively, if the liquid absorption medium used in step (v) is water or an aqueous solution so that the loaded absorption solution contains maleic acid, at least some of the maleic acid present in the loaded absorption medium can be used as or to prepare further C4+ hydrogenation feedstock for use in step (viii). In this case at least some of said resulting maleic acid can first be concentrated by removal of excess water prior to use in step (viii). Moreover maleic acid present in the loaded absorption medium, whether excess water is removed for concentration purposes or not, can be mixed with crude maleic anhydride of step (iii) for use as or to prepare the C4+ hydrogenation feedstock of step (viii).
In a preferred process according to the invention, condensation of maleic anhydride is effected in step (iii) by indirect cooling against a cooling medium selected from water and a process fluid. In an alternative preferred process condensation of maleic anhydride is effected in step (iii) in the presence of a liquid condensation medium comprising a liquid selected from maleic anhydride, monoesters of maleic acid, diesters of maleic acid, and mixtures of two or more thereof, in order to reduce the fouling of the condenser surfaces. Thus direct cooling may be carried out by spraying the liquid condensation medium into the vaporous reactant effluent stream so as to form a mixture of crude maleic anhydride and said liquid condensation medium which is used as or to prepare the C4+ hydrogenation feedstock of step (viii). Such monoesters and diesters can be derived, for example, from C1 to C4 alkyl alcohols such as methanol and ethanol. Moreover the liquid condensation medium can also contain small amounts, e.g. up to about 5 molar %, of the corresponding monoalkyl and dialkyl fumarates.
In one particularly preferred process the C4+ hydrogenation feedstock is a dialkyl maleate which is prepared by reaction of maleic anhydride with an alkyl alcohol to form a monoalkyl maleate which is then esterified with further alkyl alcohol to form a dialkyl maleate. In this case the alkyl alcohol can be methanol or ethanol and the dialkyl maleate can be dimethyl maleate or diethyl maleate. In this case the hydrogenation catalyst of step (ix) is preferably selected from copper chromite and promoted copper catalysts, such as manganese promoted copper catalysts.
In an alternative preferred process according to the invention the C4+ hydrogenation feedstock is maleic anhydride. In this case the catalyst can be any one of those proposed for the purpose in the prior art, for example one of the catalysts disclosed in one of the aforementioned U.S. Pat. Nos. 3,948,805, 4,001,282, 4,048,196, 4,083,809, 4,096,156, 4,550,185, 4,609,636, 4,659,686, 4,777,303, 4,985,572, 5,149,680, 5,473,086, 5,478,952, and 5,698,749.
Conveniently the source of gaseous oxygen is air. However, mixtures of nitrogen and air, mixtures of off gas and air, mixtures of off gas and oxygen, pure oxygen, and oxygen-enriched air may also be mentioned as the source of oxygen. The off gas may comprise that part of the residual vaporous gas that remains after absorption of further maleic anhydride in the liquid absorption medium in step (v). The source of gaseous oxygen may be used in excess so as to maintain the mixture of hydrocarbon feedstock and source of gaseous oxygen, e.g. air, outside flammable limits. Alternatively the process may be operated so that the mixture is within flammable limits.
In one particularly preferred process the hydrocarbon feedstock is a butane feedstock.
In this case the partial oxidation catalyst may comprise a vanadium-phosphorus-oxide catalyst. Such a catalyst is sometimes described as vanadyl pyrophosphate. In order to maintain the activity of the catalyst volatile organophosphorus compounds can be bled into the feed mixture to the catalytic partial oxidation zone. As some phosphorus compounds may be present in the vaporous reaction effluent stream of step (ii) of the process of the invention which may deactivate the hydrogenation catalyst of step (ix), a guard bed of a phosphorus-absorbing material, such as vanadium-containing material; conveniently a charge of spent partial oxidation catalyst, may in this case be placed in the path of the vaporous reaction stream of step (ii), after cooling thereof, in order to remove phosphorus-containing materials therefrom.
In an alternative process the feedstock comprises benzene. In this case the catalyst can be, for example, a supported vanadium pentoxide catalyst which may be modified with molybdenum oxide.
The catalytic partial oxidation zone may be of any suitable design, for example it may be a fixed bed reactor, a tubular reactor, a fluidised bed reactor, or a moving bed reactor.
Step (v) of the process of the invention may comprise absorbing vaporous maleic anhydride from the effluent stream in an organic solvent, such as a dialkyl phthalate or a dialkyl hexahydrophthalate, for example dimethyl phthalate, dibutyl phthalate, dimethyl hexahydrophthalate, or dibutyl hexahydrophthalate. Water or an aqueous solution of maleic acid can alternatively be used to absorb vaporous maleic anhydride from the effluent stream in step (v).
In the process of the invention some of the maleic anhydride recovered in step (vii) may be used as, or may be used to make, additional C4+ hydrogenation feedstock of step (viii).
It will often be preferred, during shutdown, to use an alkyl alcohol as wash liquor to wash condensation surfaces of the condensation zone to remove deposits of fumaric acid thereon and to combine the resulting solution with the C4+ hydrogenation feedstock. This procedure has the advantage of avoiding or reducing the use of sodium hydroxide and the production of aqueous byproduct streams containing maleic acid or sodium maleate which are a feature of washing procedures using water and/or sodium hydroxide solution to remove fouling deposits of maleic acid and fumaric acid from condenser surfaces.